This application relates in general to measurement and sensing of low power signals. More particularly, the invention relates to the sensing, amplification and measurement of a low power, light-based signal.
FIG. 1 illustrates a circuit 100 of the prior art for amplifying a signal from a photo diode 130. The circuit of FIG. 1 includes the photo diode 130 connected across the inputs of an operational amplifier 120. The positive input of the op amp 120 is tied to ground. A resistive load R 150 is coupled between the negative terminal and the out signal 110 of the op amp 120.
Notably, the feedback resistor R 150 has inherent thermal noise that can sometimes exceed the actual signal from the photo diode 130. The output from a resistive feedback amplifier such as circuit 100 is given in equation (1) below: EQU V.sub.out =-i R 1
where V.sub.out is in volts, i is the input signal in amperes from a signal source (such as photo diode 130) and R is the feedback resistance (such as the resistor R 150) in ohms.
A component with resistance generates thermal noise with the following RMS values: ##EQU1## where V.sub.RMS noise is in volts and I.sub.RMS noise is in amperes and where k=1.38.times.10.sup.-23 J/.degree.K (Boltzmann's constant), T is the absolute temperature in .degree.K, B is the bandwidth in Hz and R is the resistance in ohms.
Therefore, when an application requires the amplification of a very low signal from a photo diode, the prior art resistive feedback amplifier 100 sometimes proves unuseful due to excessive noise, for example.
FIG. 2 presents a circuit 200 of the art, designed to avoid this thermal noise problem. In FIG. 2, the photo diode 130 remains coupled across the inputs of the op amp 120. In place of the resistive element R 150, a capacitor 220, coupled between the negative input and the output 210 of the op amp 120, serves as the feedback element. The source of a field-effect transistor (FET) 230 is coupled to the output 210 of the op amp 120 while the drain is coupled to the negative input of the op amp 120. The gate of the FET 230 serves as a Reset signal 240.
The use of the capacitor 220 as the feedback element eliminates the noise problem of the circuit 100.
The output from an integrator such as the circuit 200 is given in equation (4) below: ##EQU2## where i is the input signal from a signal source (such as photo diode 130) in amperes, t is the time from reset to reading in seconds and C is the feedback capacitance (of capacitor 220, for example) in farads.
FIG. 3 illustrates the timing of the operation of the circuit 200 of FIG. 2. A control circuit (not shown) typically resets the integrator 200 (by means of the Reset signal 240) at twice the rate of the signal bandwidth. Just prior to each of these resets, the control circuit reads the out signal 210 and extracts the true signal.
The use of the semiconductor switch 230, however, creates its own problems in the circuit 200. The charge transfer itself from the Reset signal 240 during the resetting of the integrator 200 induces noise. To avoid this problem, the control circuit reads the out signal 210 right after releasing the reset switch 240. The control circuit then subtracts this reading from the final reading.
The noise of the photo diode 130 and op amp 120 nonetheless affect the two-reading scheme used with the circuit 200 up to the bandwidth of the system. The system bandwidth has to be much higher than the signal bandwidth in order not to distort the integration curves.
Accordingly, there is a need for a circuit for an improved detector of low levels of light without the thermal noise and other problems described above. These and other goals of the invention will be readily apparent to one of ordinary skill in the art on the reading of the background above and the invention description below.